Electric discharge device



J. E. BEGGS ELECTRIC DISCHARGE DEVICE June 8, 195 4 2 Sheets-Sheet 1Filed Aug. 16, 1950 Inventor- James E. Beggs, b3 m 3% His Attorney June8, 1954 J. E. BEGGS ELECTRIC DISCHARGE DEVICE 2 Sheets-Sheet 2 FiledAug. 16, 1950 9 J w mm fw 0% r o J n m w wzha mywuw 1., H 0 J 0 PatentedJune 8, 1954 ELECTRIC DISGHARGE DEVICE James E. Beggs, Schenectady, N.Y., assignor to General Electric Company, a corporation of New YorkApplication August 16, 1950, Serial No. 179,859

11 Claims. 1

The present invention relates to improved electric discharge devices andmethods of fabrication and exhaust.

The art of manufacturing electric discharge devices is, in general,highly developed. In spite of this the art has experienced greatdifficulty in producing tubes having satisfactory performancecharacteristics particularly as the demand for higher and higheroperating frequencies has increased. Modifications of conventional tubestructures have been employed, for example, in the present televisionbroadcast frequency bands which lie in the ranges of 54: to 88megacycles and 174 to 216 megacycles. These tubes however require somesacrifice in performance char- 7 acteristics. Other tubes, such as thedisk seal tubes widely used in high frequency applications in radar andsimilar equipment are expensive and diflicult to manufacture so thatthey have not offered, from a cost standpoint, a satisfactory tube formass production for such applications as television. With the proposedtelevision band of 475 to 890 megacycles there is a real need forelectric discharge devices possessing much better electricalcharacteristics at these frequencies and which may be manufactured inlarge quantities at a reasonable low cost.

While not limited thereto, the present invention is directed to novelstructural features and the departure from known manufacturing methodswhich may be combined to produce electric discharge devices possessingsuperior electrical characteristics with respect to gain, noise factorand transconductance. The performance of tubes manufactured inaccordance with the present invention is superior in the proposedtelevision band of 475 to 890 mega-cycles as compared with knownvarieties of disk seal tubes, for example, in the present televisionband.

The present invention involves the use of materials for the tubeenvelope which are selected for their desired electrical and mechanicalcharacteristics without regard for their thermal expansions and they arejoined together by the use of lead solders, the ductility of whichaccommodates the differences in expansion. Such solders have hithertobeen considered unsatisfactory for use in electric discharge devicesbecause of the vaporization of the solder within device during bake-outor use. The present invention also involves novel structural featureswhich essentially eliminate such vapors from the interior of the device.It is a feature of the present invention that the bonds or seals betweenthe electrically conductive and insulating parts of the envelope are ina liquid state during the outgassing and evacuation of the device sothat a high temperature bake-out is possible without subjecting theparts to high mechanical stresses which would result from cooling apreviously bonded. composite structure through such a large temperaturerange. The parts are also designed so that the soldered joints are notrelied upon for mechanical strength after the device is evacuated.

In accordance with an important aspect of the present invention thecomponent parts are shaped so that the complete tube may be assembledand supported from a single element of the envelope prior to the bondingtogether of any of the conductive and insulating parts which make up theenvelope. The spacings between the electrodes is entirely controlled bythe dimensions of the component parts which may readily be formed toaccurate dimensions. Contributing to this feature of the invention isthe positioning of the seals between radially spaced surfaces so thatsolder or sealing material does not in any way affect theinter-electrode spacing. The terminals and electrodes are alsoelectrically connected by the same solder which bonds the components ofthe envelope together so that no separate welds between the parts arerequired during assembly.

While many of the features of the present invention are usableseparately for discharge devices generally, they may be combined toproduce a greatly improved electric discharge device particularly forhigh frequency applications.

The present invention will be better understood by reference to thefollowing description of a preferred embodiment thereof considered inconnection with the accompanying drawing and. its scope will be pointedout in the appended claims. In the drawing Fig. 1 is an exploded view ofthe components of the cathode sub-assembly shown in Fig. 2 in anelevational view in section; Fig. 3 is an exploded view of the anodesub-assembly shown in Fig. 4 in an elevational View in section; Fig. 5is an exploded view of the entire electric discharge device includingthe sub-assemblies of Figs. 2 and 4; Fig. 6 is an elevational view insection of the assembled device prior to completion of the device byexhausting and formation of the bonds between the component parts; Fig.'7 is an elevational view in section of the completed device; and Fig. 8is a schematic representation of exhaust and bake-out equipment suitablefor manufacturing an electric discharge device of the present inventionin accordance with the methods of the present invention.

Before describing the embodiment of the invention with reference to thedrawing, it should be pointed out that the discharge device illustratedin Figs. 1-7, inclusive, is about five and one-half times the size of anactual embodiment of the invention which has been successfully operatedat frequencies up to 4.000 mega-cycles and with a power gain of tendecibels or more.

Referring now to the drawing, the cathode assembly'shown in Fig. 2 ismade up of the components shown in the exploded view of Fig. l. Thesupport for the cathode subassembly is provided by an insulating memberor washer I having a central aperture 2 for receiving a generallycylindrical cathode eyelet 3. Preferably the washer is of a suitableceramic but it may also be quartz or even of glass in some instances. Itwill be understood that ordinary glass will not withstand sufficientlyhigh temperature to permit the high temperatures employed in accordance1 trated at I, to receive the reduced end portion 8 of a cathodeterminal shell 9 which forms a part of the tube envelope and theelectrical terminal of the cathode. A solder ring It is placed in therecess 1 and lies between the cylindrical wall of the recess and theportion 8 of the cathode shell t. The shell s and the cathode eyelet 3are retained in the ceramic washer I by means of a spring member I Iwhich in the embodiment illustrated is in the form of a spiral havingits larger end engaging the inwardly directed flange I2 of the cathodeshell 9 and its smaller end engaging the tabs 6 which are bent outwardlyand over the upper turn of the spring II, as illustrated at E3 in Fig.2. As will be readily appreciated, other forms of spring members may beemployed for retaining the cathode and cathode terminal in position. Inorder to reduce the inductance of the cathode circuit it is preferableto provide metal fingers 3 secured to cathode eyelet 3 and having thefree ends thereof recessed between the ceramic ring I and the flange I2of the cathode terminal 9. Three equally spaced fingers may be used andif they are formed of spring material it is possible to omit the coilspring ll. The illustrated construction is preferred, however. Thecathode heater I4 isin the form of a double spiral suitably insulated bya material such as aluminum oxide in a manner well understood in theart. The heater is positioned within the cathode eyelet and the heaterleads I5 and It extend from the eyelet within the confines of cathodeshell 9.

The cathode disk t rests upon an annular sur face I? of the ring I andthis surface is accurately formed or lapped to dimensions. scribedlater, the grid cathode spacing is determined by this surface If and aconcentrically arranged annular surface I3 on the ring I from which thegrid washer is supported.

At a suitable time in the assembly of the discharge device andpreferably before assembly of the insulating and conducting parts, thesurfaces of the members, such as the member I, that are to be bonded tothe metal parts of the envelope are painted with a material whichassists in the bonding operation. In accordance with one method, knownin the prior art to be suitable for bonding ceramics or similarrefractory materials and metals, a slurry of titanium hydride formed bymixing the hydride in a suitable carrier such as acetone, amyl acetateor the like, is painted on the surfaces. Accordingly, the surface of therecess 7 of the ceramic ring I and the outer surface IQ of the flange 2dof the ceramic ring I are painted with a thin coating of titaniumhydride.

As will be de- In a similar manner the electric discharge deviceincludes an anode sub-assembly illustrated in Fig. 4 and designatedgenerally by the numeral 2|. This sub-assembly, as illustrated in Figs.3 and 4:, includes a ceramic ring 22 having a central cylindricalaperture 23 for the reception of an elongated cylindrical anode member24 which terminates in an enlarged disk-lilac portion 25 the end ofwhich provides the active anode surface. The part 2 1 extends throughthe aperture 23 and provides the external anode connection. Asillustrated, the surface 23 is painted with a titanium hydride slurryand a ring 25 of solder is interposed between the anode conductor 24 andthe wall of the recess 23. The anode is held assembled on the ring 22during the fabrication of the electric discharge device by a small ring2'! which is readily deformed into firm engagement with part 2 of theanode and in contact with the lower surface of the ring 22 as viewed inFig. l. The upper surface of the ring 2?. is recessed, as shown at 23,to provide an annular boss 29 on which the anode 25 rests and toincrease the sur face resistance of the ring between the anode and theouter flange 30 which is also painted with titanium hydride for purposesof bonding to other parts of the device. The anode conductor 24 isprovided with a longitudinal passage 3i which terminates in a radiallyextending passage 32 opening on the side wall of the anode 25. Thispassage provides for the exhaust of the discharge device, as will bedescribed in more detail at a later point in the specification.

An elevational View in section of the discharge device prior to theexhaust of the device and the bonding of the parts together is shown inFig. 6 and in Fig. 5 is shown an exploded View of these parts includingthe anode sub-assembly 2| of Fig. 4 and the cathode sub-assembly of Fig.2, the latter being designated generally by the numeral 33. Inaccordance with a feature of the invention the discharge device isdesigned so that it may be supported from a single component of theenvelope. In the particular embodiment illustrated, this part is thegrid shell or terminal designated by the number 34. The member 34 isprovided with an inwardly directed flange 35 which engages the outwardlydirected flange of the anode supporting ring 22. After assembly of theanode sub-assembly 2i into the shell 34, a grid spacer ring 36 isinserted. This ring engages the surface of the anode supporting ring 22and is provided on its upper end with an inwardly directed flange 3?which supports the grid ring 38. As illustrated, the opening 39 of thegrid ring is covered by a plurality of parallel and very fine grid wires30 which are suitably bonded to the lower surface (surface toward theanode) of the ring 38. Next, the cathode subassembly 33 is inserted andas will be apparent from an inspection of Fig. 6 the grid cathodespacing is determined by the surfaces I! and I8 of the cathodesupporting ring I, the flange 4 of the cathode and the thickness of thegrid washer 38. This unique arrangement provides for the very accuratespacing of the cathode and the easy control of this spacing inmanufacture. Next, a solder ring M is inserted and this ring liesbetween the shell 34 and a concentric outer surface of the supportingring I. If desired, an additional solder ring may be inserted prior tothe assembly of the grid and cathode subassembly in the area between thegrid ring 36 and the shell 34. However, a single ring such as II hasbeen found to be adequate.

The envelope is completed and the heater connections provided bycylindrical insulator 42 having a central cylindrical opening 43, arecess 44 and a shoulder 45. The surface of the recess 44 and the area46 just above the flange it are coated with titanium hydride. Theinsulator i2 is inserted in the cathode shell 9 and wire it of theheater is threaded through the small central opening or the ring 42. Theother heater terminal It is positioned between the ring t2 and theflange provided between the portions 8 and 9 of the cathode shell. Inthis manner, one heater lead is connected to the cathode shell 2 and theother is connected to a centrally located cathode terminal 4'! which isinserted in the opening 43 and provided with a tapered portion 48 whichengages the heater terminal I5. The lead wire I5 of the heater is wedgedagainst the edge of the aperture 43 of the insulating ring M, as clearlyvisible in Fig. 6. Solder rings 43 and 50 are placed in the recess 44and in the space between the body of insulator 42 and the cathode shell9.

With discharge device assembled as described thus far in thespecification it is ready for evacuation, bake-out and sealing oif. InFig. 8 is illustrated equipment suitable for accomplishing theseoperations in accordance with the present invention. Referring now toFig. 8, there is illustrated what is commonly termed a bell jar exhaustsystem which includes a support or table 5| having a generally planarupper surface including an exhaust port 52. Surrounding the port arethree vertically extending supports 53 on which is carried a suitablerefractory member 54 which i apertured at 55 to receive and support anelectric discharge device of the present invention. As illustrated, thedevice is supported from the grid terminal shell 34 from which theremainder of the assembled tube is entirely selfsupporting. Suitableextensions 55 of the supports 53 provide means for supporting oninverted cup-shaped metal shield 51 which is adapted to be heated byhigh frequency induction and to radiate heat to the electric dischargedevice.

Suitable lead-in conductors and terminals are provided for energizingthe heater of the electric discharge device while it is being exhausted.To this end conductors 58 and Eli are brought in through the support 5!through a suitable rubber plug or gasket fill and the conductors aresupported from the insulating member 5 3. These conductors terminaterespectively in spring terminals 6i and 62 which engage respectively theterminal 4? and the cathode terminal shell 9 which as may be seen fromFig. 6 are connected 7 with the heater terminals.

bell jar it the lower edge of which rests upon and is sealed to theupper surface of the support 51 in vacuumtight relation by a suitablerubber gasket 64. The exhaust port 52 communicates with a suitablevacuum system (not shown) through a conduit 65.

As will be described in more detail at a later point in thespecification, the electric discharge device is sealed off while Withinthe exhausted bell jar. In order to accomplish this sealing, I provide arecessed member 65 for receiving a quantity of solder which will be in amolten condition during the exhausting process and this recessed memberis movable relative to the electric discharge device so that the exhaustpassage 3| in the conductor 24 may be immersed within the molten soldercarried by the member 66. The member 56 is supported in vacuum tightrelation with respect to the support 5! and for relative movement withrespect thereto by an elongated rod til provided with a flange 68 spacedsomewhat from the point where the rod 61 emerges from the lower wall ofthe support 5|. The flange 68 is connected to a suitable boss 69 on thelower surface of the support 5! by an elongated resilient tube 10. Thetube may be in the form of a rubber hose which is clamped to the boss 69and to the flange 68 by suitable clamping rings H and 12.

A suitable source of high frequency for heating the shield or oven 57 isillustrated schematically by the coil it; as will be readily understood,this coil may be arranged to be moved into and out of operative positionwith respect to the bell jar and the shield 51.

In a typical exhaust schedule for the discharge device described indetail in the foregoing part of this specification, the bell jar isplaced over the assembly and the vacuum system placed in operation. Assoon as the pressure has been reduced to about 1 micron, which takes avery short time, in the order of 15 seconds with the system used, thehigh frequency coil '13 is energized and the shield 5i rapidly heated toa high temperature. The energy of the coil is adjusted so that theentire tube assembly reaches approximately 800 C. in a period of fourmin utes. After about a minute and a half of heating, the titaniumhydride which, as previously described, has been painted on all of theceramic surfaces which are to be soldered to metal surfaces begins todecompose and by about three minutes the solder flows over the surfacesto which the hydride has been applied. This period depends, of course,upon the particular solder employed and in accordance with an importantfeature of the present invention a very ductile solder such as lead or alead silver or lead copper alloy is used. If a pure lead alloy is usedthe melting point is 327 C. and for the alloy solders it is slightlylower. For example, a 2 ,42 silver solder melts at 305 C. After about 3minutes the entire discharge device is up to a temperature ofapproximately 800 C. and at this time the heater is energized byapplying voltage to the conductors 553 and 58. As is well understood, itis common practice to apply a higher than normal voltage to the heaterdur ing activation of the cathode and this is usually done by increasingthe voltage in steps. For a 6 volt heater, for example, the Voltageapplied may be in steps of 5, 7 and 10 with each voltage applied for aperiod of 15 seconds. This leaves the cathode energized at a voltage of10 volts for 15 seconds after the high frequency energy is turned off.This voltage supplies essentially double normal wattage to the cathodeheater. As the assembly has cooled slightly after de-energization thehigh frequency coil and the heater, the solder pot provided by themember 66 is raised to immerse the lower end of conductor 24 and asolder seal is retained in the lower end of the exhaust passage 35, asshown at id in Fig.

7. The solder pot may be lowered at once since the solder is retained inthe passage Si by capillary action. After the device has cooled below309 and all of the solder joints are in a solid condition, the devicemay be removed from the bell jar. At this time the device appears asshown in 7 and the solder rings are all melted to solder the metal andceramic parts of the envelope in hermetically sealed relationship. Thering 49 has melted and flowed around the con-- ductor 41 to seal memberto the ceramic ring 42 and to bond the heater terminal 15 to theconductor 41. The solder of the joint is illustrated at 15 and 16. Inlike manner the solder ring 58 bonds the outer surface of the ceramicring 52 to the cathode shell 9, as illustrated at H. Since the outersurface of the member 42 and the very outer edge of the lower surface ofthis member are coated with a hydride, the solder flows along the edgeof the ring and bonds the heater terminal l6. Similarly, the othersolder bond the various ceramic members to the metal members. The solderring 4| completes a bond between the grid terminal shell 34 and both ofthe ceramic rings 1 and 22 as illustrated at it and also completes theconnection from the grid to the grid terminal shell 34.

From the foregoing it is apparent that the present invention providesnot only for the simultaneous exhaust and bonding together of theenvelope parts but also the making of the electrical connection bysoldering with the titanium hyd'ide method used in accordance with thepreferred embodiment of the invention this amount essentially tocompleting the circuits between the various electrodes and thecorresponding terminals by a circuit printing. By this method allseparate welding operations, usually required for making theseconnections are eliminated.

In the foregoing description, a particular process of bonding orsoldering ceramic to metal parts has been described. This titaniumhydride method is described and claimed in the copending applications,Serial No. 36,289-Kelly filed June 30, 1948, and Serial No.36,244s-Bondley, filed June 30, 1948, both assigned to the assignee ofthe present invention. It is to be understood that the present inventionis not limited to this par ticular method of bonding and that in itsbroader aspect the invention may be applied to any method in which thesoldering operation may be carried out in a vacuum. It is also possiblethat the ceramics be metalized prior to the assembly of the tube so thatonly the soldering operation is completed during the fabrication andexhausting of the tube within the bell jar.

In the foregoing description, the insulating parts have been describedas ceramic. There are a large number of ceramics which are suitable forelectric discharge devices and they may be selected in accordance withtheir electrical and mechanical characteristics. The high alumina bodiesand the silicate bodies are particularly suitable. Th invention is notlimited to materials strictiy classified ceramics since quartz, forexample, may be used. It is of particular advantage to use high meltingor softening point insulating materials which will stand the hightemperature bake-out which the present process makes possible. It isalso not necessar to many aspects of the present invention that leadsolders be employed. It is a particular advantage, however, to use asoft solder so that the parts are not required to have matching thermalexpansion characteristics. This permits the metal parts and the ceramicparts to be selected for their other desired characteristics. in theparticular embodiment Llustrated, the metal parts of the envelope havebeen formed of a copper or copper coated steel. In an extreme case thepart i has been mad of quartz and parts a and 34 of cop-per. it isnecessary to give some attention to the amount of solder available inthe joint and its relation to the amount of mismatch between expansioncharacteristics of the component parts to he joined. It is alsoimportant to note that the present technique of making electricdischarge devices permits a very high tern erature of bake-out duringexhaust, and that during this time all of the joints between theinsulating and conducting members of envelope are in a fluid state. Thismeans that the bonds themselves are not required to Withstand atemperature change from the high temperatur of 800 (3., in the examplegiven down to the point who e the solder solidifies, namely at about 327C. for pure lead.

From the foregoing description it will be apparent that adjacent partsof the envelope bear d tctly against one another so that the solder 3snot called upon to withstand any appreciable mechanical. force after thedevice is The anode assembly is the one exception and as previouslydescribed the ring 21 has a press fit with anode 2 so that the load isremoved from the joint between the ring 22 and the part This permitsoperation of the device at temperatures approaching the softening partof the soft solder.

It is also s gnificant that the bonds or solder joints are in generalmade between circumferentiaily spaced surfaces so that the jointsthemves not enter into the electrode spacing which is determined. by themechanical dimen sions of the members in the stack. The cathode to gridspacing particularly is very accurately controlled the dimensions of theceramic ring i, particularly surfaces ii and i8 thereof, the thic nessof he cathode disk t and the thickness he grid washer or disk 33. Theresilient support for t e cathode takes care of any expansions encoi.ered during operation or" the device. construction also eliminatesdirect communi ation between the solder of the bonds and. theinter-electrode space so that detrimental vapor from the soft soldersdoes not enter into this region and contaminate the interior of thedevice.

While I have described a particular embodiof my invention, it will beapparent to those skilled in the art that changes and modifications maybe made without departing from my inven tion in its broader aspects andI aim in the appended claims to cover all such changes and modificationsas fall within the true scope and spirit of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A cathode subasseinbly for an electric discharge device comprising adisk-like insulator having a central aperture therethrough, a bearingsurface surrounding said aperture on one surface of said insulator, agenerally cylindrical cathode structure positioned in said aperture andincluding an outwardly extending bearing surface engaging the bearingsurface of said insulator, a hollow generally cylindrical cathodeterminal having an inwardly directed flange engaging the opposite sideof said insulator and spring means interposed between said inwardlydirected flange and said cathode structure for urging said cathodestructure into engagement with the bearing surface of said insulator andelectrically connecting said cathode and said cathode terminal.

2. A cathode subassenibly for an electric discharge devic comprising aninsulating' member forming a part of the envelope of the device andhaving an aperture therethrough, a bearing surface surrounding saidaperture on one surface of said insulator, a cathode structurepositioned in said aperture and including an outwardly extending bearingsurface engaging the bearing surface of said insulator, a cathodeterminal shell bonded to said insulating member, means electricallyconnecting said cathode with said terminal shell and spring meansinterposed between an opposed surface of said insulator and said cathodestructure for urging said cathode structure into engagement with thebearing surface of said insulator.

'3. An electric discharge device comprising a hollow generallycylindrical conductive grid terminal, a cathode assembly joined to oneend of said terminal and including an insulating support having twopositioning surfaces occupying accurately determined relative positions,a cathode surface positioned from one of said bearing surfaces, a gridmember electrically connected to said grid terminal and spaced from saidcathode from the other of said bearing surfaces, an anode assemblysealed to the other end of said grid terminal and including aninsulating support on which the anode is supported in accuratepositional relationship and a spacer member within said grid terminalinterposed between said grid and the insulating support of said anodeassembly for determining the grid anode spacing.

4. An electric discharge device comprising a cathode assembly includingan insulating support having two positioning surfaces occupyingaccurately determined relative positions, a cathode surface positionedfrom one of said bearing surfaces, a grid member spaced from saidcathode from the other of said bearing surfaces, an anode assemblyincluding an insulating support on which the anod is supported inaccurate positional relationship and conductive means connected withsaid grid and providing a terminal therefor, and a cylindrical gridterminal of conducting material surrounding both said insulatingsupports and bonded thereto to join said anode and cathode subassembliestogether.

5. An electric discharge device comprising a hollow generallycylindrical conductive grid terminal, a cathode assembly joined to oneend of said terminal and including an insulating support having twopositioning surfaces occupying accurately determined relative positions,a cathode surface positioned from one of said bearing surfaces, a gridmember electrically connected to said grid terminal and spaced from saidcathode from the other of said bearing surfaces, an anode assemblysealed to the other end of said grid terminal and including aninsulating support on which the anode is supported in accuratepositional relationship and a spacer member within said grid terminaldetermining the spacing between said anode assembly and said cathodeassembly.

6. An electric discharge device comprising an envelope includingelectrically insulating and conducting members positioned in alternaterelation in the wall of the envelope and providing mutually insulatedexternally accessible terminals of the device, a filamentary memberwithin the device having terminal portions received respectively betweendifferent adjacent insulating and conducting members and solder jointsconnecting the terminal portions to said conducting members and bondingsaid conducting members to the adjacent insulating members.

7. An electric discharge device comprising an envelop including aplurality of insulating and conducting members having generally circularsymmetry, said members being positioned in a stack with the relativepositions determined by opposed bearing surfaces in an axial directionto determine the spacing between the elements, a plurality of electrodeswithin said envelop and positioned respectively from certain of saidinsulating members and connected with different ones of said conductingmembers, adjacent ones of said members having closely spaced radialsurfaces, and a body of solder between the closely spaced radialsurfaces for hermetically sealing the members together without affectingthe interelectrode spacing.

8. An evacuated envelope comprising a plurality of metal members and aplurality of insulating members having mismatching thermal expansioncharacteristics with respect to said metal members, said members beingstacked to provide the envelope wall with the metal members providingmutually insulated terminals, and ductile bonds between continuoussurfaces of adjacent members, said bonds each comprising a layer of alead solder.

9. An evacuated envelope comprising a plurality of metal members and aplurality of insulating members having mismatching thermal expansioncharacteristics with respect to said metal members, said members beingstacked to provide th envelope wall with the metal members providingmutually insulated terminals, ductile bonds between contiguous surfacesof adjacent members, said bonds each comprising a layer of a leadsolder, and means including abutting surfaces on adjacent members forrelieving the duotile bonds of mechanical force due to atmosphericpressure on the exterior of said envelope.

10. An evacuated envelope including an insulating member and a pair ofconducting members spaced apart and mutually insulated by saidinsulating member, said members having mismatching thermal expansioncharacteristics over the temperature range to which the envelope is tobe subjected in use, said members having abutting surfaces determiningthe spacing between said metal members and absorbing the compressiveforce due to atmospheric pressure on the exterior of said envelope, andductile bonds between other surfaces of said members hermeticallysealing said members together, said bonds including a lead solder.

11. An evacuated envelope including an insulating member and a pair ofconducting members spaced apart and mutually insulated by saidinsulating member, said members having mismatching thermal expansioncharacteristics over the temperature range to which the envelope is tobe subjected in use, said members having abutting surfaces determiningthe spacing between said metal members and absorbing the compressiveforce due to atmospheric pressure on the exterior of said envelope, saidmembers also having closely spaced circumferentially opposed surfacesand ductile lead solder bonds between said latter surfaces hermeticallysealing said members together.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,229,436 Beggs Jan. 21, 1941 2,244,358 Ewald June 3, 19412,446,269 Drieschman Aug. 3, 1948 2,461,303 Watson Feb. 8, 19492,462,921 Taylor Mar. 1, 1949

